Current-to-pneumatic converter and amplifier systems produce a pneumatic pressure proportional to a given electrical current. This produced pneumatic pressure may be referred to as a signal pressure. The signal pressure may be amplified, both in pressure and volume, and fed to a pneumatic actuator used to position a valve in a valve positioner system. Such a system is described in U.S. Pat. No. 6,957,127, issued Oct. 18, 2005.
In a typical two-stage valve positioner, the second stage is used to amplify both the flow capacity and pressure range of the output since a typical current-to-pneumatic converter may have a low flow and minimal pressure gain. Pneumatic control valve positioner manufacturers have traditionally used two types of pneumatic amplifiers. One type is a spool valve design. The second type is a pneumatic relay, which may be called a relay. A pneumatic relay is a pneumatic amplifier that converts a small pneumatic input pressure signal into a larger pneumatic output signal comprising both pressure and flow. Pneumatic relays are used to supply pressurized air to a pneumatic valve actuator, such as a piston, to position or throttle a valve.
Inherent to the design of the relay is an end-loading of a supply plug on a corresponding supply seat during steady state operation of the relay. This end-loading is due to the pressure drop across the plug and the force due to a supply plug spring. During dynamic response of the relay, signal pressure must be increased sufficiently to overcome this end-loading before any additional output flow is established. This change in signal pressure with no corresponding output flow is referred to as a flow deadband. Elevated venting back pressure has been known to force open the supply plug, allowing supply pressure to flow into the vent area, which, in turn, can significantly increase venting time.